1. Field of the Invention
This invention relates to concrete foundations, and more particularly, to reinforced concrete pier foundations useful for the support of tall, heavy and/or large towers which may be used to support power lines, communication systems, street lighting and signals, bridge supports, commercial signs, freeway signs, ski lifts and the like, and especially wind turbines which are subject to very high upset forces.
2. Background of the Invention
Various different forms of foundations utilizing some of the general structural and operational features of the instant invention have been known. However, these previously known foundations do not include some of the basic features of the instant invention. The combination of features incorporated in the disclosed embodiments of the instant invention enable a heavy duty foundation to be formed in situ or be precast and transported to the construction site, and be placed in position in a manner not requiring the use of large drilling rigs or pile drivers. The present invention is particularly useful in locations where the ground water is high and/or the depth clearance for placement of the foundation is low. The foundation of the present invention is capable of resisting very high upset loads and in a manner independent of the concrete of the foundation experiencing alternating localized compression and tension loading.
In a conventional concrete pier foundation the concrete bears the compressive loads and the contained reinforcing bars (rebar) are sized to bear the tensile loads. Anchor bolts are typically utilized solely for the purpose of attaching the supported structure to the foundation. The anchor bolts are typically placed within a reinforcing steel (rebar) matrix or cage using a removable template at the top and a separate anchor plate at the bottom of each bolt to prevent anchor bolt pullout. The entire module is poured in concrete. Conventional foundations typically resist overturning movement by their weight.
Further, when the foundation is loaded by the structure supported thereon, the foundation is subjected to varying tensile and compressive loads with there being a boundary at the bolt anchor plates where the loading on the concrete alternates from a compressive load to a tensile load depending upon the various forces on the supported structure. The tensile load from the overturning moment of the supported structure is applied near the top of the foundation by the anchor bolts and subjects the large portion of the foundation below the point of application to tension. The large foundation typically requires a great amount of reinforcing steel and a large amount of concrete to encase the reinforcing steel. Extensive labor is also necessary to assemble the reinforcing steel matrix and fill the volume of the foundation with concrete and fix the anchor bolts. A typical cylindrical foundation also requires the use of a large drill to excavate the hole.
My two prior U.S. patents, U.S. Pat. Nos. 5,586,417 and 5,826,387, hereby incorporated by reference in their entirety as if fully set forth, are hereinafter sometimes referred to as xe2x80x9cmy prior patentsxe2x80x9d. My prior patents disclose concrete pier foundations under high unit compression which eliminate the necessity for large quantities of reinforcing steel bars (rebar or tension bars) and substantially reduce the amount of concrete necessary. Therefore, the cost of these foundations is substantially less when compared to conventional designs. Furthermore, the foundations of my prior patents simplify the placement of the supported structure on the foundation and eliminate alternating cyclical compression and tension loading on the foundation, thereby substantially reducing fatigue. Also, these patented foundations allow for the replacement of the tower anchor bolts in the unlikely event of bolt failure and allow for removal of the upper four to five feet of the foundation in the event such action is desired for decommissioning purposes.
The foundations of my prior patents are preferably in the shape of a concrete cylinder. The outer boundary shell of the concrete may be formed by corrugated metal pipe (CMP). The inner boundary, preferably in large hollow cylinder foundations, also may be formed by CMP of a lesser diameter. Elongated high strength steel bolts are positioned around the periphery of the concrete cylinder and extend from a peripheral anchor plate or ring near the bottom of the cylinder vertically up through the concrete portion of the foundation to a peripheral connecting plate or flange adjacent the upper end of the structure. The bolts are received in xe2x80x9chollow tubesxe2x80x9d to isolate the bolts from the concrete. The steel bolts preferably serve as anchor bolts for attachment of the base flange of the supported tower structure, and the anchor bolt pattern may thus be determined by the bolt hole pattern on the lower base plate or flange of the supported tower. The steel bolt pattern may also be engineered into the construction of the foundation by a removable template.
The xe2x80x9chollow tubesxe2x80x9d are preferably elongated plastic tubes or sleeves which encase the bolts substantially through the entire vertical extent of the concrete and prevent the bolts from adhering to the concrete. Then, the bolts can be tensioned after the concrete has hardened and cured in order to post-tension the entire concrete foundation. Alternatively, the elongated bolts can be wrapped in plastic tape, or otherwise sheathed to prevent the bolts from adhering to the concrete during curing and allow the bolts to stretch under tension over the entire operating length of the bolt through substantially the full vertical extent of the concrete. There is typically no conventional rebar reinforcing steel in the foundation, except perhaps in large foundations where a small amount of incidental rebar may be used to stabilize the bolts during construction of the foundation.
The center of a large hollow cylindrical foundation according to my prior patents is filled with excavated soil and then capped. Excavation for the foundation may be done using widely available, fast, low cost excavating machines instead of relatively rare, slow, costly drills necessary for conventional cylindrical foundations.
The foundations of my prior patents use the mechanical interaction with the earth to prevent overturning instead of the mass of the foundation typically used by other foundations for turbine towers and other large structures subject to high upset forces. The high strength anchor bolts are tightened to provide heavy tension on the foundation from a heavy top flange through which the bolts pass to the anchor flange or plate adjacent the bottom of the foundation, thereby post-stressing the concrete in great compression and placing the entire foundation, between the heavy top plate or flange and lower anchor plate or flange, under high unit compression loading. The bolts are tightened so as to exceed the maximum internal tension expected from overturning forces of the tower structure on the foundation. Therefore, the entire foundation withstands the various loads with the concrete thereof always remaining in compression and the bolts always in static tension. The concrete pier foundations of my prior patents may be formed in situ in a remote location or be formed in a precast concrete section placed under heavy compression.
To describe a portion of the foundation structure incorporated in the foundation of the present invention, reference is now made to FIGS. 1, 2 and 2A which are taken from my prior patents. In these Figures, the numeral 10 generally designates the pier foundation. The foundation 10 includes inner and outer upstanding corrugated metal pipe (CMP) sections 12 and 14. The outer section 14 is initially placed within a hole or excavation 16 formed in the ground 18 and rests upon the bottom of the excavation 16. The inner section 12 is then placed and positioned within the excavation 16. The interior of the inner CMP 12 is partially back filled and the excavation 16 outwardly of the outer CMP 14 is partially back filled to stabilize the CMP sections generally in position within the excavation and relative to each other.
The foundation 10 additionally includes a series of tensioning bolts 20 and 21 spaced circumferentially about the annulus defined between sections 12 and 14. The tensioning bolts are preferably in side-by-side pairs which are spaced radially from the center of the foundation. In a typical embodiment, the inner ring of bolts 20 has a slightly shorter diameter than the outer ring of bolts 21. Forty-eight tensioning bolts 20 and forty-eight tensioning bolts 21, or a total of ninety-six, are provided. The rings of bolts have diameters which are several inches apart and which generally equal about 12 feet.
The lower ends of the bolts 20 and 21 are anchored to a lower annular plate or anchor ring 22, sometimes also called an embedment ring, which preferably may be constructed of several circumferentially butted and joined sections. The anchor or embedment ring 22 is radially spaced relative to the inner CMP section 12 preferably by utilizing circumferentially spaced horizontal and radially extending positioning bolts 24 threaded through nuts 26 secured to the under side of the anchor ring 22 at points spaced circumferentially thereabout. Further, the bolts 20 and 21 have all but their opposite ends slidingly received through hollow tubes, such as PVC pipes 30, which are sized to receive and loosely grip bolts 20 and 21 but still permit free movement therethrough. The hollow tubes or PVC tubing 30 need not extend through the entire vertical height of concrete 68, but only through as much of the central portions and extending as close to the top and bottom as to allow the tensioning bolts to extend evenly through the concrete during post-tensioning.
The PVC pipes 30 and other suitable tubing or isolating mechanism serve to allow bolts 20 and 21 to move relatively freely through the concrete after curing so as to allow post-tensioning of the elongate bolts 20 said 21. In addition, rebar wraps 28 are used and secured to the tubes 30 associated with outer bolts 21 at approximately five foot intervals along the vertical extent of the bolts 21 in order to maintain the bolts longitudinally straight during the pour of concrete.
The upper ends of the bolts 20 are supported from a template 32 which consists of upper and lower rings 34 and 36 or ring sections secured together. Upwardly opening radial channel members 38 and mounting blocks 40 received in the channel members 38 are clamped between the upper and lower rings 34 and 36 through the utilization of upper and lower nuts 42 and 44 threaded on the bolts 20 and 21. The inner ends of the radial channel members 38 are joined by a center circular plate 46. The inner portions of the radial channel members 38 include lateral stabilizers 45 in the form of inverted channel members downwardly embracingly engaged thereover and equipped with opposite side set screws 47 clamp engaged with the corresponding channel members 38. The depending flanges of the channel members 45 are slotted for stabilizing engagement with adjacent upper edge portions of the inner pipe 12 while the outer ends of the channel members 38 include threadingly adjustable channel member feet 50 abutingly engageable with the ground 18. A cylindrical form plate 52 is clamped about the upper end of the outer pipe 14 and has its opposite ends secured together in overlapped relation.
Thereafter, concrete 68 may be poured to the bottom of each of the radial channel members 38 and to the top of each of the blockout bodies 64. After the concrete 68 has hardened, the upper nuts 42 are removed and the entire template 32 including the upper and lower rings 34 and 36, the channel members 38 and attached feet 50 are lifted up from the bolts 20 and 21 and the form plate 52, the blockout bodies 64 being exposed from above by removal of the template 32 to thus allow removal of the blockout bodies 64.
When the concrete 68 has sufficiently hardened, it must be determined that the groove 70 is level. If groove 70 is not level, a coating of high compression hardenable grout should be placed within the groove 70 to achieve a levelness for the tower base. Further, even if groove 70 is level, it may be desirable to place grout in the groove 70 as illustrated in FIG. 8 of my U.S. Pat. No. 5,586,417. The nuts 44 are removed or threaded downwardly on the bolts 20 and 21 at least xc2xe inch. A tower (not shown) to be supported from the foundation 10 is thereafter lowered into position with the upper exposed ends of the bolts 20 and 21 upwardly received through suitable bores formed in the inner and outer peripheries of the heavy annular plate or base flange of the tower and the lower lug defining portion of the base flange seated in the groove or grout trough 70. Initially the upper nuts 42 are again threaded down onto the upper ends of the bolts 20 and 21 and preferably torqued to about 50 foot pounds. The nuts 42 are thereafter sequentially torqued (in a predetermined pattern of tightening) preferably to about 600 foot pounds which places each of the bolts 20 and 21 under approximately 40,000 pounds tension at approximately ⅓ the stretch limit of the bolts 20 and 21.
As illustrated in FIG. 1 and as described in my U.S. Pat. No. 5,586,417, column 8, in lines 37-43, the backfill within the inner pipe 12 may be completed to substantially ground level and provided with a poured concrete cap 86. The cap 86 may be sloped toward the center thereof and provided with a draining conduit 88, and a conduit 90 for electrical conductors (not shown) also may be incorporated in the foundation 10.
By placing the bolts 20 and 21 under high tension, the cylindrical structure comprising the concrete 68 is placed under heavy unit compressive loading from the upper end thereof downwardly to a level adjacent the lower end of the cylindrical structure, and the unit compressive loading is considerably greater than any upset tensional forces which must be overcome to prevent upset of the tower and foundation 10. As a result, the concrete foundation 68 is always under compression and never subject to alternating compression and tension forces.
The perimeter weighted foundation of the present invention incorporates the above described post-stressed annular foundation of my prior patents as a pier pedestal or pedestal section of the foundation of the present invention. The present invention differs from the structures disclosed in my prior patents in that the foundation of the present invention includes an expanded base positioned adjacent the bottom of the pedestal section which has a cylindrical perimeter wall that is substantially larger in diameter than the cylindrical wall of the pedestal section and that extends to a depth substantially below the bottom of the pedestal section. The expanded base also includes a generally horizontal disk-like spread section which interconnects the bottom of the pedestal section and the top of the perimeter wall or perimeter wall section. This combination greatly increases the foundation""s resistance to an overturning moment by use of skin friction along the exterior sides of the perimeter wall and pedestal section and by the additional weight and size concentration at the perimeter of the foundation, including the back fill on top of the horizontal spread section.
The foundation of the present invention also eliminates alternating cyclical compression and tension loading stresses, thereby substantially increasing internal stiffness and reducing fatigue. Vertically extending anchor bolts in the pedestal section are sleeved in PVC or equivalent shielding to prevent bolt bonding with concrete for compression of the concrete as disclosed in my prior patents. Radially extending bolts are embedded horizontally in the disk-like spread section for reinforcement. The radially extending horizontal bolts are also sleeved in PVC or equivalent sheathing for high compression loading of the annular spread section. It has not been found necessary to post-tension the cylindrical perimeter wall and post-tension bolts are preferably omitted from this section. However, such bolts may be included as part of the present invention if high unit compression of the exterior perimeter wall section is desired.
The foundation of the present invention reduces time, and reduces cost by allowing excavation by conventional backhoes and/or track excavators. Further, the tensionless pedestal section can be assembled and poured at a fabrication yard and shipped as a precast component to the foundation site. The corrugated metal pipe (CMP) forms provide reinforcing steel and establish a boundary for the concrete pour. The bolt holes in the CMP provide support and positioning for the horizontal radial arrangement of the laterally extending bolts in the spread section, along with eliminating the need for temporary perimeter forms and reinforcing steel supporting chairs and blocks. The foundation may be poured continuously (monolithically) or sequentially, first the perimeter wall, then the spread section and finally the pedestal section. Alternatively, the pedestal section can be poured first, then the perimeter wall and finally the spread section in two separate pours.
The exterior cylindrical or perimeter wall section is made of concrete preferably formed between two corrugated metal pipes (CMPs). In one embodiment, the outer CMP of the perimeter wall is a few feet taller than the inner CMP of the perimeter wall in order to define the outer circumference of the horizontal spread section. Initially, a circular excavation having a diameter slightly greater than the diameter of the perimeter wall is dug to the depth of the pedestal section. An annular shaped trench for forming the perimeter wall is excavated to a depth of the desired bottom of the perimeter wall. In this embodiment, the initial circular excavation to the depth of the pedestal section leaves a raised perimeter section whose inner diameter is greater than the outer diameter of the pedestal section and whose outer diameter extends to the outer wall of the circular excavation. Then when the annular shaped trench for forming the perimeter wall is dug, a slightly raised annular berm-like earth ring is left inside the perimeter wall trench and above the bottom of the excavation for supporting the pedestal section.
The two CMPs forming the exterior and interior of the perimeter wall are placed into the perimeter wall excavation. Sand cement slurry is placed between the exterior of the CMPs and the sides of the perimeter wall excavation. The CMPs to form the pedestal section are then placed on the bottom of the interior circular shaped excavation and plumbed and properly centered within the interior of the CMPs for the perimeter wall section.
A first or lower set of radially extending bolts extend through the interior and exterior CMPs of the pedestal section and pass preferably to a mid-point between the interior and exterior CMPs of the perimeter wall section, at which point, the lower bolts are bent 90 degrees to extend downwardly between the two CMPs forming the perimeter wall. The horizontally extending portion of the radially extending lower bolts is jacketed to prevent engagement with the subsequently poured concrete. Threading is included on the terminal end of the lower bolts passing through the interior CMP of the pedestal section to receive suitable post-tensioning fasteners such as a large washer and nut. The opposite ends of the lower bolts which project downwardly between the CMPs of the perimeter wall do not include a protective sleeve so as to be able to form a strong bond with the subsequently poured concrete to strengthen the perimeter wall.
Progressively larger diameter circles of high strength cable are placed on top of the lower layer of radially extending bolts and secured at the cross-over intersections. These cables referred to as xe2x80x9ctendonsxe2x80x9d, are used instead of rebar to strengthen the horizontal spread section. The tendons are placed such that each successive tendon diameter preferably is approximately two feet greater than the preceding tendon diameter. The opposed ends of each tendon are preferably overlapped by approximately 60xc2x0 or about eight feet.
Located above the first or lower layer of radially extending bolts is a second or upper layer of radially extending PVC sleeved bolts passing through the inner CMP of the pedestal section and out to the outer CMP of the perimeter wall section. The opposed ends of the upper radially extending bolts are secured interiorly of the pedestal section and exteriorly of the perimeter wall by suitable fastening, such as a large washer and nut, for post-tensioning of these bolts. These bolts also have a series of increasing diameter circles of tendons on their upper surface which are preferably aligned with the tendons on the lower layer of radially extending bolts.
Once the upper layer of radially extending bolts are secured to the circularly extending tendons, concrete is poured for the perimeter wall section and the steel reinforced slab forming the spread section. The annular earth berm below the underside of the spread section forms a depending annular shoulder on the bottom surface of the spread section adjacent the base of the pedestal section. Electrical and communication conduits are installed. The surrounding soil is backfilled to a finish grade from the edge of the excavation to the exterior CMP of the pedestal section. Concrete is then poured between the CMPs for the pedestal section to within about two feet of the finish grade. The radially extending bolts passing through sleeves in the spread section are post-tensioned preferably to about fifty percent of their yield strength. This places the concrete in the spread section under high unit compression load. The interior of the pedestal is backfilled. No compaction of the soil is required.
As in my prior patents, the center drain pipe, form rings, styrofoam block outs and leveling nuts are installed. The remaining pedestal portion is poured along with the concrete tower floor. The form rings and templates are removed for subsequent securing of the supported tower into place, and the vertical anchor bolts are post-tensioned to place the pedestal section in high unit load as described in my prior patents.
In an alternate embodiment of the present invention, the perimeter wall section and spread section can be formed as separate components. In addition, the annular shoulder on the bottom of the spread section can be omitted if desired. Instead of the interior and exterior CMPs for forming the perimeter wall having different heights, the interior and exterior CMPs of the perimeter wall section are about the same height. The two layers of post-tensioning bolts extend horizontally through the interior and exterior CMPs of the perimeter wall section as well as the interior and exterior CMPs of the pedestal section. The concrete for the perimeter wall section and spread section can then be poured separately and the generally horizontal bolts embedded in the spread section can be post-tensioned as described in the previous embodiment.
The concrete foundation of the present invention can be utilized on all sizes of wind turbines currently being commercially marketed and constructed, which range in size from 500 KW to 2500 KW. These towers range in height from the ground to the hub from 40 to 100 meters, and the weight of the towers, including the tower, blades, and nacelle (containing gear boxes, generators, etc.) connected atop the tower, range from 150,000 pounds to 700,000 pounds. The overturning moments for the larger units are from 6,000,000 to 50,000,000 ft-lbs. Further, by expanding the outer diameter of the perimeter wall in the present invention to 50 feet and increasing the thickness of the spread section to 6 feet, it is believed that the concrete foundation of the present invention could be designed to handle overturning moments up to as much as 100,000,000 ft-lbs, which would be encountered with tower heights in the neighborhood of 400 feet.
It is believed that the concrete foundation of the present invention is adaptable to very large overturning moments and is capable of supporting larger turbines than the tensionless foundations of my prior patents, which currently have depth limitations imposed by economic factors of around 35 feet. While the pedestal portion of the present invention can always be the same size as the tensionless hollow cylinder of my prior patents, the perimeter foundation of the present invention has the ability to extend the diameter of the perimeter wall to even greater diameters. The depth of the perimeter wall could approach 30 feet and the moment resistance of such a wall depth coupled with the increased diameters could support any foreseeable size tubular wind turbine tower.
Accordingly, it is a first object of the present invention to provide a post-tensioned concrete foundation which has increased resistance to overturning moments, especially for larger wind turbines and other large tower structures.
A further object of the present invention is to provide a post-tensioned concrete foundation which maximizes resistance to overturning by situating weight concentration and skin friction resistance at the perimeter of the foundation.
A still further object of the present invention is to provide a post-tensioned concrete foundation which has high upset resistance by including an expanded base to a pedestal section of the foundation.
Another object of the present invention is to provide a foundation in accordance with the preceding objects and in which the expanded base includes a cylindrical perimeter wall section spaced a substantial distance outward from the wall of the pedestal section and a horizontal spread section which interconnects the bottom of the pedestal section and the top of the perimeter wall section.
Still another object of the present invention is to utilize excavated soil as backfill to add weight to the pedestal section, the top of the spread section and the top of the perimeter wall section to resist overturning forces while using inexpensive on-site materials.
Yet another object of the present invention is to provide a post-tensioned concrete tower foundation having an expanded base which can be constructed without temporary independent forms and shoring.
Still a further object of the present invention is to provide a post-tensioned concrete foundation for weak soils such as with shallow ground water in which the top of the expanded base of the foundation can be positioned at, above, or below the ground water surface.
Yet a further object of the present invention is to provide a post-tensioned concrete foundation adaptable for areas with deep frost depths in which the top of the expanded base of the foundation is set below the frost line.
Still yet another object of the present invention is to provide a perimeter weighted post-tensioned concrete foundation in which the major components, the pedestal section, the spread section and the perimeter wall section, can be pre-assembled and hauled to a site for placement into a foundation excavation or poured concrete can be cast in place.
Still yet a further object of the present invention is to provide a perimeter weighted post-tensioned concrete tower foundation having a pedestal section and an expanded base which will conform to conventional forms of manufacture and provide a foundation that will be economically feasible, long lasting and relatively trouble free in use.